Abstract: An acoustic monitor assembly for monitoring the particulate content of a liquid, comprising, a body having an electrically conductive portion, the body contacting a liquid whose content is to be monitored in use, a magnetic field generator for generating a magnetic field to which the electrically conductive portion of the body is exposed, the magnetic field generator being positioned such that magnetic particulates in the liquid are drawn on to said body, and a signal generator for inducing eddy currents which oscillate at an acoustic frequency in the electrically conductive portion of the body and in response to which the body is caused to vibrate, and a monitoring for observing the vibration condition of the body so as to provide an indication of the content of said particulates which are drawn onto the body in the liquid.